Maybe I'm misunderstanding something, but C12/13 look to me to be inside a loop. The diagram also, to me, underscores PJWs tendency to have a high order LF rolloff. FWIW I use 34s and like them. But as general preamps, modified a bit. I've avoided using their 'disc' (LP) input stage.
BTW One of the reasons for tweaking the LF rolloff is that such a filter tends to affect higher frequencies as well. So may sound like an oddity at other frequencies. Similarly, the presence of low level LF affects how we hear higher components.